Orthopedic and dental implants are used for a variety of joint and teeth replacements and to promote bone repair in humans and animals, particularly for hip and knee joint and tooth replacements. Although many individuals experience uncomplicated healing and restoration of function, there is also a high rate of complications, estimated at 10-20% for total joint replacements. The majority of these failures and subsequent revision surgeries are made necessary by failure at the implant-bone interface. In addition, implants used as anchorage devices for orthodontic tooth movement have an estimated 40% failure rate and subsequent placement of additional implants is made necessary because of failures at the implant-bone interface.
Orthopedic and dental implants are made of materials which are relatively inert (“alloplastic” materials), typically a combination of metallic and ceramic or plastic materials. Previous approaches to improve the outcomes of orthopedic implant surgeries have mainly focused on physical changes to the implant surface designed to increased bone formation. These approaches include using implants with porous metallic surfaces to promote bone ingrowth and spraying implants with hydroxyapatite plasma. Approaches using dental implants have also included the use of topographically-enhanced titanium surfaces in which surface roughness is imparted by a method such as grit blasting, acid etching, or oxidation.
Also in an effort to promote osseointegration, implant surfaces have undergone major alterations. For example, short peptides containing an arginine—glycine—aspartic acid (RGD) sequences have been attached to implant surfaces because cells utilize RGD sequences to attach to the extracellular matrix. Investigators have attempted to recreate this cell attachment to the modified implant surface but this strategy has resulted in only modest increases in implant osseointegration and mechanical fixation. Alternatively, in an attempt to stimulate blood vessel ingrowth around implants their surfaces have been coated with a coating containing the angiogenic growth factor VEGF. Implants soaked in saline solutions have been marketed as a means to increase implant osseointegration, with little or no data to substantiate the claims.
Another strategy employed to stimulate osseointegration is to nano-texture the implant surface. The rationale behind this strategy is that texturing increases surface area and therefore prevents the implant from “sliding” against cells in the peri-implant environment. In clinical trials, however, nano-texturing does not result in measureable benefits.
The use of protein-based approaches to stimulate implant osseointegration has also been under intense investigation. Recombinant Bone Morphogenetic Proteins (BMPs) induce robust bone formation in skeletal fractures and they have also been employed in an effort to stimulate direct bone formation around implants. While in vitro results have been encouraging, in vivo data are less convincing. Recombinant BMPs inhibit osteogenic differentiation of cells in the bone marrow cavity and consequently, are contraindicated for implant osseointegration. See Sykaras et al. (2004) Clin Oral Investig 8(4): 196-205; and Minear et al. (2010) Journal of Bone and Mineral Research 25(6): 1196-207. The use of BMPs has been associated with increased incidence of heterotopic ossifications and uncontrolled inflammation and more recent metadata analyses demonstrate an increased risk of cancers as well.
Wnt proteins form a family of highly conserved secreted signaling molecules that bind to cell surface receptors encoded by the Frizzled and low-density lipoprotein receptor related proteins (LRPs). The WNT gene family consists of structurally related genes which encode secreted signaling proteins. These proteins have been implicated in oncogenesis and in several developmental processes, including regulation of cell fate and patterning during embryogenesis. Once bound, the ligands initiate a cascade of intracellular events that eventually lead to the transcription of target genes through the nuclear activity of β-catenin and the DNA binding protein TCF (Clevers H, 2004 Wnt signaling: lg-norrin the dogma. Curr Biol 14: R436-R437; Nelson W J, Nusse R 2004 Convergence of Wnt, beta-catenin, and cadherin pathways. Science 303: 1483-1487; Gordon M D, Nusse 2006 Wnt signaling: Multiple pathways, multiple receptors, and multiple transcription factors. J Bioi Chem 281: 22429-22433).
Wnts are also involved in a wide variety of cellular decisions associated with the program of osteogenesis. For example, Wnts regulate the expression levels of sox9 and runx2, which influences the commitment of mesenchymal progenitor cells to a chondrogenic or an osteogenic cell fate. Wnts influence the rate of differentiation of osteoprogenitor cells. In adult animals there is abundant evidence that Wnt signaling regulates bone mass. For example, gain-of-function mutations in the human Wnt co-receptor LRP5 are associated with several high bone mass syndromes, including osteopetrosis type I, and endosteal hyperostosis or autosomal dominant osteosclerosis. Loss-of-Wnt-function mutations cause low bone mass diseases including osteoporosis-pseudoglioma. Increased production of the Wnt inhibitor Dkk1 is associated with multiple myeloma, a disease that has increased bone resorption as one of its distinguishing features. For further details, see, S. Minear et al., Wnt proteins promote bone regeneration. Sci. Transl. Med. 2, 29ra30 (2010); Zhao et al., Controlling the in vivo activity of Wnt liposomes, Methods Enzymol 465: 331-47 (2009); Popelut et al., The acceleration of implant osseointegration by liposomal Wnt3a, Biomaterials 31 9173e9181 (2010); and Morrell N T, Leucht P, Zhao L, Kim J-B, ten Berge D, et al. (2008) Liposomal Packaging Generates Wnt Protein with In Vivo Biological Activity. PLoS ONE 3(8): e2930.
It has been shown that combining Wnt proteins with lipid vesicles (liposomes) produced a Wnt formulation (Morrell et al., 2008, supra; and Zhao et al., 2009, supra) with biological activity (Minear et al., 2010, supra; and Popelut et al., 2010, supra). The biological activity of soluble wingless protein is described in van Leeuwen et al. (1994) Nature 24: 368(6469): 3424. Biochemical characterization of Wnt-Frizzled interactions using a soluble, biologically active vertebrate Wnt protein is described by Hsieh et al. (1999) Proc Natl Acad Sci USA 96(7): 3546-51. Bradley et al. (1995) Mol Cell Bioi 15(8): 4616-22 describe a soluble form of Wnt protein with mitogenic activity. The use of liposomal Wnt proteins to enhance osseointegration is described in U.S. Patent Publication No. 20120115788.